Method of making transducer

ABSTRACT

A silicon transducer including a silicon frame with one or more lands extending from a diaphragm or the like. The lands are interconnected by two thin strips formed integrally with the lands. The strips are essentially the transducer. The transducer is constructed by etching a boron doped wafer with a mixture of catechol, ethylene diamine and water.

This is a division of application Ser. No. 043,466, filed May 29, 1979.

BACKGROUND OF THE INVENTION

This invention relates to transducers, and more particularly toelectrical semiconductor transducers of the acoustic type.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a pressuretransducer, including a flexible diaphragm supported in a frame of thesame material as the diaphragm, first and second lands extending fromone surface of the diaphragm, and a plurality of semiconductor filamentsstretched between and formed integral with the lands, the device beingso constructed that changes in the configuration of the diaphragm causecorresponding changes in the tension of the filaments and hence changesin natural and/or resonant frequency of vibration.

The principle on which the transducer functions is analogous to thevariation with tension of the resonant frequency of a stretched string.The resonant frequency of such a transducer is a direct function of anapplied force with temperature variation a second order effect. Theelectrical output of the transducer is in a form particularly suitablefor signal processing by logic circuitry or by a microprocesser.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings which illustrate exemplary embodiments ofthe present invention:

FIG. 1 is a perspective view of a semiconductor resonant filamenttransducer;

FIG. 2 is a top plan view of the transducer of FIG. 1;

FIG. 3 is a cross-sectional view of the transducer taken through aresonant filament; and

FIG. 4 is a schematic diagram of an oscillator drive circuit for usewith the transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, a transducer 11 is shown which is formed byselective etching of a silicon wafer doped with boron.

In FIG. 2 parts that remain after the etching process are shown.

The fabrication of selectively etched silicon devices depends upon thephenomenon of inhibition of certain etching steps by a concentration ofdopant boron higher than 4×10¹⁹ atoms/cc. There is an abrupt change inthe etch rate from that normal for undoped silicon to substantially zeroat this boron concentration level so that the thickness of an unetchedregion is defind precisely by the boron diffusion depth. The process ismore fully described in U.K. Specification No. 1,211,496 (J. C.Greenwood 6). The wafer is doped through a mask with boron in thoseareas in which etching is not required, and the wafer is then etchedwith a mixture of catechol, ethylene diamine and water to form thetransducer structure shown in FIGS. 1 and 2.

The transducer 11 includes a pair of lands 12 (as opposed to grooves)protruding from a silicon diaphragm 13 supported in a rectangularsilicon frame 10 and linked by a pair of filaments 14 when pressure isapplied. In practice of course a plurality of such devices is disposedon a silicon wafer.

As, contrary to conventional semiconductor fabrication techniques, bothmajor surfaces of the semiconductor wafer are etched, means must beprovided for handling a process wafer by its rim portions only. Thus thetransducers formed on a wafer must be disposed towards the centralregion leaving the rim portion free.

In the apparatus used for the etching process, the silicon wafer onwhich the devices are to be formed is mounted by its rim portion on aglass carrier and is then sealed via an O-ring against a shoulder at oneend of a tube. A clamp ring or gland nut metal with a reverse thread onthe tube secures the wafer in position. Etch solution can then be pouredinto the cup thus formed and act on one side only of the wafer. In someapplications the glass carrier may be replaced by a vacuum chuckarrangement.

In a typical transducer fabrication process, the silicon wafer iscleaned in hydrofluoric acid, caro's acid and water and is then treatedto a boron diffusion from both sides. The front of the wafer is maskedwith an evaporated aluminum layer with the reverse face masked andetched in a phosphoric acid etch to define the face of the diaphragm.The aluminum coating has a photoresist on it, and is etched with aphosphoric and/or nitric acid mixture. The silicon is etched by plasmaetching to a depth greater than has been rendered insoluble in aselective etch by the boron diffusion. This is done on both sides of theslice, which is then etched in a selective etch to define the firstconfiguration of the device.

The various etching techniques will be apparent to those skilled in theart, but the following rules should be observed:

1. The etch rate of catechol-diamine-water is substantially slower inthe <111> crystallographic direction than in any other. To a firstapproximation the rate in the <111> direction can be regarded as zero.

2. A concave face tends to be opened up to give a hollow bounded by theslowest etching <111> (octahedral) faces; thus a pinhole in a protectiveoxide coating on a <111> orientation slice gives rise to a squarepyramidal etch pit.

3. A convex face tends to give a solid bounded by the fastest etchingfaces which are the 24 <331> faces.

4. An irregularity, such as might be caused by faulty masking, in a<111>face tends to be straightened out whereas the same sort ofirregularity in a fast etching face does not.

5. The cleanest <331> fast etching faces are obtained when one edge onlyis adjacent to another fast etching face, the other edges being adjacentto unsoluble material or <111> planes. Irregular shapes result fromother alignments although not every combination of adjacent faces hasbeen tried.

6. On <100> slices a variety of corner shapes can be obtained by puttingcompensating spikes on the mask. The angle of the spikes does not appearto be critical although a 1 in 3 slope gives good results. The length ofa spike is related to the etch time, which is determined by thethickness of the slice. Normally the etch time should be greater than isneeded just to reach the other side, so that any irregularities arecleaned up. If the etch time is 20% greater than is needed to reach theother side, a nearly square corner is obtained by making the length ofthe spike 20% longer than the thickness of the slice. If no spike isused, the corner is chamfered. Intermediate sized spikes giveintermediate results.

7. If a part of the surface is to be undercut, care has to be taken thatthis process is not stopped by <111> faces. For example if a bridge isto be undercut on a <100> orientation slice, the bridge must be at anangle to the <111> faces and must be sufficiently narrow.

In FIG. 3, which is a cross section of part of the transducer, we seeone of the resonant strips at 20, which is spaced from the lower surface21 of the device. On the lower surface 21 there are the devices drivingelectrode 22, its pick-up electrode 23, and a guard electrode 24therebetween.

In use the transducer, which functions as a pressure gauge, is mountedby its rim or frame against a source of pressure to be measured. Thefilaments are excited at their resonant vibrational frequency, e.g. by acircuit of the type shown in FIG. 4, this frequency being determined bythe pressure difference across the diaphragm. Changes in pressure causecorresponding changes in the tension of the filament 14 and hencechanges in their resonant frequency.

Conveniently the transducer may be excited electrostatically, thecircuit of FIG. 4 being intended for this purpose, as this providessubstantially no damping of the filament.

To reduce the capacitive coupling between input and output to a minimum,two resonant strips, as shown in FIGS. 1 and 2, are used, those beingdriven in antiphase. The preamplifier FET's are mounted on thetransducer itself. The circuit of FIG. 4 shows amplifier stages whichgive an overall gain of about 100. The second of these stages is an AGCstage and the third is a phase splitter with unity gain. The fourthstage is an optional stage to get an increased amplitude output.

What is claimed is:
 1. The method of making a silicon pressuretransducer, said method comprising the steps of: selectively borondoping both faces of a silicon wafer; etching one face of said wafer todefine a cavity within a frame forming a closed loop and two landsspaced from each other and from said frame, said diaphragm connectingsaid lands; and selectively etching the other face of said waferopposite said groove so as to define a silicon diaphragm to which bothof said lands are fixed, said etching steps leaving at least twofilaments having a first pair of adjacent ends fixed to one land and asecond pair of adjacent ends fixed to the other land in a plane spacedfrom said diaphragm.
 2. A process as claimed in claim 1, in which saidselective etching is effected with a mixture of catechol, ethylenediamine and water.